The present invention relates to a hydraulic system used in a hybrid-type cargo handling industrial vehicle.
In recent years, hybrid type cargo handling industrial vehicles have been developed. The hybrid type industrial vehicle has a power source such as a gasoline engine and another power source constituted by a battery and a motor. For example, Japanese Laid-Open Patent Publication No. 2005-298163 discloses an industrial vehicle having a generator motor that is driven by an engine and generates electricity, and a cargo handling hydraulic pump that operates in conjunction with the generator motor. In this configuration, the generator motor is driven by the engine and generates electricity. The generated electricity is stored in a battery, and the stored electricity is used for driving a drive motor. Also, the generator motor may be used as a cargo handling motor by using the electricity of the battery. That is, the generator motor is used for actuating a cargo handling pump to generate hydraulic pressure for handling cargos. Further, since the generator motor and the cargo handling hydraulic pump operate in conjunction with each other, hydraulic pressure needed for handling cargos can be generated by driving the engine to drive the generator motor.
On the other hand, Japanese Laid-Open Patent Publication No. 7-315792 discloses a hydraulic circuit for an industrial vehicle equipped with a power steering apparatus to improve the steering operability when the vehicle is moving. The hydraulic circuit has a bypass passage that supplies hydraulic oil supplied to a steering circuit by a steering hydraulic pump to a cargo handling circuit. When the load of a cargo handling operation is small, the cargo handling hydraulic pump, which has a large capacity and receives large load, is stopped, and hydraulic pressure generated by a steering hydraulic pump, which has a small capacity is used for cargo handling. This configuration reduces the energy consumption by the power source.
In a hybrid type industrial vehicle in accordance with Japanese Laid-Open Patent Publication No. 2005-298163, the cargo handling hydraulic pump is actuated when the generator motor operates. Thus, when the generator motor is driven by the engine, the cargo handling hydraulic pump is operated in conjunction with the operation of the generator motor, so that hydraulic pressure for handling cargos is generated even if no cargo handling operation is performed. That is, the energy of the engine is redundantly consumed for driving the generator motor, which degrades the energy efficiency for driving the industrial vehicle.
A hybrid type industrial vehicle with a power steering apparatus has a steering hydraulic pump in addition to a cargo handling hydraulic pump. When the steering hydraulic pump is operated, hydraulic oil is supplied to the steering circuit, which is independent from the loading circuit. Therefore, the energy consumption may be suppressed by applying, to a hybrid type Industrial vehicle, the hydraulic circuit disclosed in Japanese Laid-Open Patent Publication No. 7-315792, that is, a hydraulic circuit that uses hydraulic pressure generated by a steering hydraulic pump to perform cargo handling.
However, the hydraulic circuit disclosed in Japanese Laid-Open Patent Publication No. 7-315792 is designed for supplying hydraulic pressure generated by a hydraulic pump to a cargo handling circuit. Thus, it is impossible to cancel the reduction in the energy efficiency caused by a configuration unique to the hybrid type, that is, by a configuration in which the cargo handling pump is actuated in conjunction with the generator motor.